Flexibly suspended heat exchange head for a DUT

ABSTRACT

A thermoconductive module to control the temperature of a DUT including a top surface having an area and a topography comprising, in combination, a heat exchange surface for interfacing and engaging with the top surface of the DUT, a plurality of individually moveable elements arranged throughout the area of the top surface of the DUT for moving the heat exchange surface to contour the heat exchange surface to map the topography of the top surface of the DUT and means in thermal communication with the heat exchange surface for producing the heat transfer between the top surface of the DUT and the heat exchange surface.

The present invention relates broadly to the field of integrated circuit(IC) or chip manufacture and use and particularly to a device forprecisely controlling and measuring the temperature of a device undertest (DUT).

During manufacture by the chip maker, chips typically undergo threeseparate test cycles: (1) in-process testing, such as continuousmonitoring of sheet resistivities, junction depths, and other pertinentdevice parameters, such as current gain and voltage breakdown; (2) apreliminary electrical testing called burn-in; and (3) a detailed finaltesting for reliability and performance to grade or sort the chips. Thepresent invention relates to improvements in the last type of testing.

The final testing of chips is one of the more expensive and timeconsuming stages of the manufacturing process. Automatic high speedtesting is practically mandatory to the final testing of modern chipsbecause a large number of complex tests are required to check even thesimplest types of circuits.

After burn-in, it is conventional for the chips to undergo a number offunctional tests to evaluate their performance. One by one, each chip issubjected to a series of long and short functional tests. The number andcomplexity of these functional tests varies from chip maker to chipmaker. Long functional testing of digital memory chips generallyinvolves the pattern testing of each chip on an individual basis.Commonly used routines are checkerboard patterns of 1s and 0s orfloating of a 1 or 0 from cell to cell while the adjacent cells aremaintained in the opposite state. For larger memories, the generation ofthese test patterns requires a larger number of functional tests.Generally, the time required for adequate pattern testing increases at arate which is proportional to the square of the number of bits ofstorage in the digital memory chip. As the bit storage capacity of adigital memory chip increases, the time required for adequate patterntesting increases at an exponential rate.

Short functional testing of chips involves the testing of each chip onan individual basis to determine whether it meets the specs set down inthe data sheet, e.g. operating speed, and voltage and currentparameters. These so-called short functional tests generally requiremuch less testing time than pattern testing. Both the long and shortfunctional tests have heretofore been performed by chip makers invarious sequences and at various temperature levels. After thefunctional tests are completed, the chips that have satisfactorilyundergone all tests are subjected to quality control testing.

In this third stage, the functional tests are designed to test the chipsat a constant temperature, usually the junction temperature. For chipswith low power dissipation, eg <1 watt, maintaining the temperatureconstant by convection, flowing a fluid (air stream) across a DUTsurface, is usually sufficient.

As transistor densities and counts (per chip) continue to increase, thepower dissipation (P_(D)) of a chip increases markedly. P_(D) alsoincreases, proportionately, with increasing clock rate (for the commonCMOS devices). The vast majority of digital systems change theirinternal states in synchronism with a square wave or clock signal commonto the entire chip. Performance or useful work performed by a chip pertime (R), is usually directly proportional to the clock rate orfrequency. Current and proposed design P_(D)'s are becoming prohibitive(the chips are getting too hot). Chips are designed to operate in highlytemperature variable environments. The heat generated by a chip affectsits temperature and thus feedback exists. It is always desirable tooperate the chip at a constant internal temperature (junctiontemperature). Typically, this internal temperature is set to be lessthan the maximum allowable to allow for the violability and powerconsumption goals of the chip design. With the testing of the currentand the expected proposed chip designs, the total heat impinging on thechip increases significantly (due either to external temperatureincreases or to increases in the system clock frequency).

When a chip is performance tested at its maximum capacity and maximumsystem clock frequency, it is necessary to control the ambienttemperature to maintain the junction temperature of the chip constant inorder to provide a reliable frame of reference or standard against whicheach chip is tested. When a chip is tested, it is referred to as adevice under test (DUT).

Therefore, as the ability of chip manufacturers to reduce the physicalsize of chips has improved, the power dissipation in the chips somanufactured has accordingly increased. As a result, when the DUTs aretested, it has become increasingly necessary to provide some form ofcooling to maintain the DUT at a constant temperature, usually itsjunction temperature.

Generally, the prior art systems are not capable of preciselycontrolling the DUT temperatures at >3-5 watts of power dissipation.

Presently, there are two major problems in precisely controlling thetemperature of a DUT. In the third stage of testing as described above,the power dissipation inherent in current chips (and future chips) ishigh. The corresponding heat generated must be removed substantiallysimultaneously (heat sink). In correlation with the rapid heat removalis the requirement of precise monitoring and control of the DUTtemperature at the desired test temperature.

Therefore, one major problem faced is to establish a superior heattransfer relationship between the DUT and a heat exchange module whichengages the DUT.

The other manor problem is to measure and control the temperature of theDUT. With regard to this latter problem, various approaches are known inthe prior art for measuring heat flow. One such approach is illustratedin U.S. Pat. No. 3,720,103 which relates to a heat flux meter. In thatdevice, thermocouples are used to measure the temperature differencebetween two surfaces. The sensed temperature difference controls aheater which is adjusted so that heat flow between the surfaces isprevented. The first surface is shielded from the environment to preventheat flow therefrom to this surface. This device, however, is notsuitable for measuring the performance of a cooling device such as aheat sink or heat transfer device used in a semiconductor module forcooling a semiconductor chip or the like.

Another method is illustrated in U.S. Pat. No. 3,745,460. In thisapproach, a current pulse is fed into the semiconductor causing heat tobe generated therein. The detected time interval between cessation ofthe pulse and detection of maximum heat transfer leads to adetermination of the thermal resistance.

A further method is described in U.S. Pat. No. 4,396,300. The apparatusincludes an electric heater for heating a block which surrounds andengages part of the tube. A liquid is pumped through the tube and athermistor is used to measure the fluid temperature. A pressure dropsensor is provided to sense the drop in pressure across the block. Thesensed data is transferred to a computer for computing the heat transferresistance. Like the other approaches mentioned above, this method toois not suitable for determining the effectiveness of a heat transferdevice used in a module to cool a DUT.

However, these other problems of accurate and effective temperaturecontrol of a DUT during ‘burn in’ were overcome in my earlier issuedU.S. Pat. Nos. 5,126,656; 5,164,661; 5,315,240 and PCT PublicationWO94/22029 which are hereby incorporated by reference in theirentireties into this disclosure. That is, my earlier work and inventionsfor the control and measurement of a DUT during ‘burn in’ are applicableto the control and measurement of the temperature of a DUT during thefunctional testing (third stage) of a DUT.

The present invention overcomes the one major problem heretoforedescribed and is directed to a device which establishes a superiorheat-transfer relationship between a DUT and a heat-exchange device.Although the invention will be described with reference to chips (ICs),the thermoconductive module is also applicable for the testing of otherdevices such as hybrids, multi-chip modules, dc/dc converters, etc.

Broadly, the invention comprises a thermoconductive module whichprovides for superior conductive heat transfer from a DUT. The modulecomprises a housing having a heat exchange chamber. A flexible heatexchange plate is secured to the housing and interfaces with the exposedsurface of the DUT. The plate is in thermal communication with the heatexchange chamber. The plate is biased outwardly from the housing suchthat the plate maps the topography of the surface of the DUT. A DUTsensor in the housing measures the temperature of the DUT. A sensor inthe housing measures the temperature of the heat exchange fluid. Basedon the readings from these two sensors, the flow of the heat exchangefluid is controlled.

In a preferred embodiment, the surface is secured to the housing by atleast one flexible web bellows. To ensure that the heat exchange platemaps the contour of the DUT surface, a vacuum is drawn in the interfacebetween the heat exchange plate and the engaged surface of the DUT.

Broadly, the invention comprises a thermoconductive module whichprovides for superior conductive heat transfer from a DUT. The modulecomprises a mixing assembly wherein fluids at different temperatures andflow rates can be introduced and combined. The fast response time of themodule is due to the mixing of the fluids within the module. Withoutthis mixing in the module, the time lag would be unacceptable in manyapplications. A heat exchange surface, which includes a heat exchangechamber, is biased outwardly from the mixing assembly. The heat exchangeassembly has a flexible heat exchange plate which engages the topsurface of a DUT. The heat exchange plate is in thermal communicationwith the heat exchange chamber. The plate is biased outwardly from theheat exchange chamber such that the plate maps the topography of thesurface of the DUT. In a preferred embodiment, a vacuum is drawn in theinterface between the heat exchange plate and the surface of the DUT toensure maximum surface contact. A sensor in the heat exchange assemblywhich is thermally isolated from the heat exchange fluid measures thetemperature of the DUT. The sensor in the heat exchange fluid measuresthe temperature of the heat exchange fluid. Based on the readings fromthese two sensors the flow of the heat exchange fluid is controlled tomaintain the temperature of the DUT at a target temperature, typicallythe case temperature but also the junction temperature if desired.

Although the preferred embodiment will be described with reference todrawing a vacuum between the heat exchange plate and the surface of theDUT, depending upon the device being tested and the testing conditions,a vacuum is not always necessary. However, even if the vacuum is notused during the test cycle, the vacuum concept is also advantageouslyemployed with the module to pick up devices by means of the vacuum. Thisallows devices to be engaged and transferred and disengaged without amechanical device other than the vacuum feature of the module.

In a preferred embodiment, a plurality of spring biased pins engage theheat exchange plate. This allows the plate to contour to the surface ofthe DUT.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front, partially sectional view of a thermoconductive moduleof the invention;

FIG. 2 is a front view of a mixing assembly;

FIG. 3 is a top view of the mixing assembly of FIG. 2;

FIG. 4 is a front view of a pin block assembly;

FIG. 5 is a bottom view of the pin block assembly of FIG. 4;

FIG. 6 is an illustration of the interface between a heat exchange plateand the top surface of a DUT; and

FIG. 7 is a block diagram of a system embodying the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 1, a thermoconductive module is shown generally at 10.The module comprises a support plate 12 having an inlet 14 and an inlet16. Fluids, e.g. water, may be introduced into the inlets at the same ordifferent temperatures. Also secured to the support plate are outlets 18and 20 (not shown) to remove heat exchange fluid from the module as willbe described. Lastly, secured to the support plate 12 is a vacuum outlet22.

Depending from the support plate 12 is a mixing assembly 24, also shownin FIGS. 2 and 3. The mixing assembly comprises a mixing chamber 26 incommunication with the inlets 14 and 16, a conduit 28 and a distributionhead 30. Return channels 32 and 34 are formed in the outer surface ofthe walls of the mixing assembly 24 and communicate with the outlets 18and 20.

Referring to FIGS. 1, 4 and 5, depending from the mixing assembly 24 isa pin block assembly 40. The pin block assembly 40 is characterized by acentral through aperture 42 and four equally spaced feed conduits 44,shown most clearly in FIG. 5. The pin block assembly 40 furthercomprises cylindrical recesses 46 in which are received springs 48.Lastly, pins 50 are received in the cylinders 46 and are biasedoutwardly from the pin block assembly by the springs 48.

Referring to FIG. 1, a flexible wall 52 comprising bellows 54 is securedat one end to the support plate 12. Secured to the flexible wall at itsother end is a heat exchanger 60 having an upper wall 62 and a reducedlower cylindrical wall 64. Secured to the lower wall 64 is a flexibleheat exchange plate 66. A gasket 68 is secured to the heat exchangeplate 66. In an alternative embodiment, where a vacuum is not necessaryin the test cycle or the module is not to be used for moving devices,the gasket 68 is not required.

The heat exchanger 60 defines with the pin block assembly 40, a heatexchanger chamber 70. As shown, the pins 50 pass through the heatexchange chamber 70 and contact and bias outwardly the heat exchangeplate 66. The pins 50, in addition to ensuring flush engagement of theheat exchange plate 66 with the top surface of the DUT, also providebaffling for the heat exchange fluid flowing therethrough as will bedescribed. Pins 50 also provide for additional heat transfer from theplate 66 to the pins 50 to the fluid.

Extending through the heat exchange plate is a thermocouple 72 which isbiased outwardly. The thermocouple, as described in my aforementionedpatents, is thermally isolated from the heat exchange chamber. A vacuumline 74 is sealingly secured to the heat exchange plate 66 and extendsthrough the central aperture 42 and the mixing assembly 24 and connectsto the vacuum outlet 22. Extending into the heat exchange chamber is athermocouple 76. The thermocouple 76 is attached to an arm 77 which inturn is secured to the bottom of the pin block assembly 40. The pairedwires for the thermocouple 76 return through the return channel (shownas a single line for clarity) and into the outlet 18. The paired wiresare removed from the outlet 18 in a seal tight manner.

The paired wires for the thermocouple 72 travel through the vacuum line74 and then are removed (not shown) from the vacuum line in a seal tightmanner after they pass through the support plate 12.

Referring to FIG. 6, a DUT 80 is represented as a three dimensionalsolid body. For reasons of clarity, connecting pins in the substrate onwhich the chip is mounted are not shown. The DUT is seated in a testerof the manufacturer's specification which performs the functional teston the DUT.

As shown, when the heat exchange plate 66 engages the DUT, it maps thetop surface. Additionally, a vacuum is drawn at the interface forimproved thermal performance and for device pick up if required.

Referring to FIG. 7, the thermoconductive module 10 of the invention isshown schematically in a system. The heat exchange plate 66 of themodule is interfaced with a DUT as shown in FIG. 6. The vacuum line 22communicates with a vacuum source 90 and the vacuum is controlled byvalve 92. The paired wires from the thermocouples 72 and 76 (shownschematically in FIG. 1) are shown collectively as 94 and communicatewith a programmable logic controller 100. The heat exchange fluid inletand outlet conduits 14, 16 and 20 communicate with a source of heatexchange fluid 110 and have associated valves 102, 104, 106 (not shown)and 108 respectively. These valves communicate with the controller 100via lines 112, 114, 116 (not shown) and 118.

The heat exchange supply 110 includes first and second reservoirs (notshown) to maintain separate sources of heat exchange fluid at separatetemperatures. Also, the return conduits 18 and 20 flow to a reservoirfor later recycling and/or reuse.

As hereinbefore mentioned, the use of the two thermocouples, one tomeasure the temperature of the DUT and the other to measure thetemperature of the body of the heat exchange device to control thetemperature of the DUT based on the readings from the thermocouples, isset forth in detail in my aforementioned patents and publication. In thepresent application, the thermocouple 76 reads the temperature of theheat exchange fluid rather than the temperature of a heat exchangedevice. Also, the control of the flow rates of heat exchange fluidsbased on sensed temperatures is well within the skill of the art.

The operation of the invention will be described with reference to a DUT68 with a power dissipation of 0 to 100 watts. The DUT has a top surfacearea 82 of approximately 6.45 cm² (one in²). The DUT must be maintainedat a junction temperature of 85° C. for 5 minutes.

The DUT 80 is seated in a tester as shown in FIG. 7. Thethermoconductive module 10 is placed into contacting engagement with thetop surface 82 of the DUT 80. Any suitable device may be used to effectthis placement such as a robotic hand, pneumatic rods, etc., it beingunderstood (referring to FIG. 7) that the conduits 14, 16, 18, 20, and22 are flexible. The heat exchange fluid used for this illustrativeembodiment is water. The heat exchange plate 66 is preferably stainlesssteel 25.4 μm (0.001″) thick with an outside surface coating of aprecious metal, such as gold, in a thickness of about 50 millionths. Thethermocouple 72 engages the top surface of the DUT and measures itstemperature.

The bellowed wall 52 allows the heat exchange plate 66 to move withreference to the support plate in a gimbal-like fashion. As shown inFIG. 6, the pins bias the plate 66 to ensure maximum surface contactbetween the heat exchange plate and the top surface of the DUT 80. Thepogo pins 50 allow the heat exchange plate 66 to map the topography ofthe surface 82. For this specific example described herein, the pogopins are uniformly arrayed such as shown in FIG. 5 and each has a springtension of approximately 0.139N (0.5 ounces).

A vacuum is drawn through the conduit 42 in a range of 98.2 kPa (29 inHg). Water flows through the inlet 14 at a flow rate of about 1 gpm andat a temperature of about 60° C. Water flows through the inlet 16 at atemperature of about 20° C. and a flow rate of 1 gpm. The water is mixedin the mixing chamber 26, flows through the conduit 28 and into thedistribution head 30. The mixed water then flows through the four feedconduits 44 and into the heat exchange chamber 70. The water leaves theheat exchange chamber, flows through the return channels 32 and 34 andthen to the outlets 18 and 20. Once the system has reached equilibrium,the tester commences the functional testing of the DUT.

Based on the readings from the thermocouples 72 and 76, the flow ratesand temperatures of the water through the inlets 14 and 16 will changeto ensure that the DUT is maintained at its junction temperature. Forthis specific example, the flow rate of the cooler water would increasefrom the initial flow rate just described while the flow rate of thewarmer water would decrease from the initial flow rate just described.The flow rates will vary during the test period.

Although described in reference to water as the heat exchange fluid,other fluids such as silicone oils, flourinets, glycols, etc. may beused.

What is claimed is:
 1. A thermoconductive module to control thetemperature of a DUT including a top surface having an area and atopography comprising, in combination: a heat exchange surf ace forinterfacing and engaging with the top surface of the DUT; a plurality ofindividually moveable elements arranged throughout the area of the topsurface of the DUT for moving the heat exchange surface to contour theheat exchange surface to map the topography of the top surface of theDUT to ensure maximum surface contact between the heat exchanger surfaceand the top surface of the DUT; and means in thermal communication withthe heat exchange surface for producing heat transfer between topsurface of the DUT and the heat exchange surface, with the mapping ofthe topography of the top surface of the DUT by the heat exchangesurface maximizing the heat-transfer relationship between the heatexchange surface and the top surface of the DUT.
 2. The thermoconductivemodule of claim 1 wherein the heat exchange surface is formed by aflexible heat exchange plate having one and another side, with the oneside interfacing and engaging with the top surface at the DUT, with theflexible heat exchange plate being separately formed from the pluralityof individually moveable elements which contact the other side of theflexible heat exchange plate.
 3. The thermoconductive module of claim 1wherein the producing means comprises a heat exchange chamber forreceiving a heat exchange fluid, with the flexible heat exchange platedefining a wall of the heat exchange chamber.
 4. The thermoconductivemodule of claim 3 wherein the plurality of individually moveableelements are arranged in an array throughout the area of the top surfaceof the DUT.
 5. The thermoconductive module of claim 4 furthercomprising, in combination: a block assembly moveable relative to theDUT, with the plurality of individually moveable elements mounted to theblock assembly.
 6. The thermoconductive module of claim 5 wherein theindividually moveable elements are mounted for movement relative to theblock assembly perpendicular to the top surface of the DUT.
 7. Thethermoconductive module of claim 6 wherein the individual moveableelements are biased relative to the block assembly.
 8. Thethermoconductive module of claim 5 wherein the individually moveableelements are slidably mounted relative to the block assembly forindependent reciprocating movement.
 9. The thermoconductive module ofclaim 5 wherein the plurality of individually moveable elements provideheat transfer between the heat exchange surface and the producing means.10. The thermoconductive module of claim 1 further comprising incombination: means for drawing a vacuum between the top surface of theDUT and the heat exchange surface.